1. Field of the Invention
This invention relates to fabrication of semiconductor devices and, more specifically, to a procedure for fabrication of field oxide.
2. Brief Description of the Prior Art
In the fabrication of semiconductor devices and particularly integrated circuits and with reference to FIG. 1a, active or moat regions 23 are generally isolated from each other. This is accomplished by deposition of a hard mask 27, generally in the form of a nitride, over silicon dioxide which grows over the wafer of semiconductor material with apertures being formed in the nitride in a predetermined pattern, generally by etching, down to the oxide on semiconductor material in the pattern of the isolation region. The wafer is then heated in an oxidizing ambient, preferably by steam, to grow a field oxide 21 in the unmasked or apertured regions of the mask. As the dimensions of the semiconductor components and particularly the moat regions and the field oxide regions have decreased and particularly when the active regions have dimensions of 0.4 microns and less, a nitride mask is required which has less bending capability than was used in the prior art, such prior art masks generally being about 1300 Angstroms in thickness over about 400 Angstroms of oxide. This has generally been accomplished by providing a thicker nitride mask of at least 1600 Angstroms and above and a thinner oxide mask of about 200 Angstroms. A problem with the use of the thicker nitride and thinner oxide mask is that, since there is essentially no bending of the nitride mask, the oxide formed, which has approximately twice the volume of the silicon which was used in its formation, grows straight up and creates regions of very high stress, particularly in those corner regions 29 where the enlarged volume of field oxide contacts the nitride mask. The nitride is then removed using a wet chemical etch process, this process initially using HF to remove any oxide that has been formed on the surface of the nitride during the thermal oxidation with subsequent removal of the nitride. Since the regions of high stress etch much more rapidly in HF than those regions not under stress and since HF is an etchant for silicon oxide, the oxide in the regions of high stress will be removed much more rapidly than the oxide formed on the unstressed or much less stressed field oxide. In addition, whenever the regions of oxide which are under high stress are subjected to HF during later processing, this will etch at a much faster rate than will any other unstressed and less stressed oxide regions. The result is that, after nitride removal, the field oxide 21 which defines the moat or active region 23 contains an escalloped region 25 where the high stress oxide existed and was removed by the HF as shown in FIG. 1b. This results in a reduced usable flat surface region 13 on the field oxide as described.
A problem resulting from the reduced field oxide surface is that, in the subsequent processing, it is often necessary to provide a narrow slot over the field oxide which, with present technology, will have a pitch of 1.5 microns or less. Since the dimensions of the slot approach the width of the field oxide surface region 21, it is important to maintain the flat portion 13 of the surface of the field oxide as wide as possible to provide tolerance for some misalignment of the slot. This tolerance requirement is diminished in the prior art due to the etching away of the portion 25 of the field oxide 29 under stress. It is therefore apparent that a technique is desirable which will retain the portion of the field oxide now removed and which will relieve the stress on the field oxide.